Cast nickel base alloy

ABSTRACT

An improved cast nickel base alloy has a combination of stability, high-temperature stress rupture strength and hot corrosion resistance primarily through control of such phases as sigma, eta, and the carbides. Control is achieved through the balance of the elements A1, Ti, Mo, W, Co, Cr and C in the proper relationships with a Ni base in the substantial absence of Fe.

United States Patent Earl W. Ross Cincinnati, Ohio [21] Appl. No. 772,796

[22] Filed Nov. 1, 1968 [45] Patented Oct. 26, 1971 [73] Assignee General Electric Company [72] Inventor [54] CAST NICKEL BASE ALLOY 3 Claims, No Drawings [52] US. Cl 75/171,

148/32.5,148/162 [51] Int. Cl C22c 19/00 [50] Field of Search 75/171,

[56] References Cited UNITED STATES PATENTS 2,951,757 9/1960 Brown 75/171 3,385,698 5/1968 MacFarlane et a1. 75/171 3,467,516 9/1969 Barker 75/171 Primary Examiner Richard 0. Dean Attorneys-Derek P. Lawrence, E. S Lee, 111, Lee H. Sachs,

Frank L. Neuhauser, Oscar B. Waddell and Melvin M. Goldenberg l cxsr NICKEL sass ALLOY The invention described and claimed in the US. Pat. application herein resulted from work done under united States Government contract FA-SS-66-6. The United States Government has an irrevocable, nonexclusive license under said application to practice and have practiced the invention claimed herein, including the unlimited right to sublicense others to practice and have practice the claimed invention for any purpose whatsoever.

Advancing technology and development of improved power-producing apparatus such as the gas turbine engine has identified the need for stronger alloys which are stable at relatively high operating temperatures such as up to 1,800 F. and yet can withstand the corrosive atmospheres in which they are intended to operate. Although a number of alloy systems including those based on the refractory metals have been evaluated for such applications, the nickel base alloy remains the type presently most widely used in such difficult applications.

One high-temperature nickel base alloy application of particular interest is the cast form of the alloy. However, known nickel base alloys in cast forms either are relatively weak or unstable during longtime operation or have insufficient resistance in hot corrosive atmospheres particularly in the l,500 F.-l,800 range.

It is a principal object of this invention to provide an improved cast nickel base alloy having an unusual combination of strength and stability for longtime operation at elevated temperatures coupled with hot corrosion resistance.

A more specific object is to provide such an alloy of improved stability and having a stress rupture life in the as-cast condition of at least 25 hours under a stress condition of 27,500 psi. at l,800 F.

These and other objects and advantages will be more apparent from the following detailed description and typical representative exampleswithin the broad scope of the appended claims.

It has been recognized that a cast nickel base alloy having an improved combination of high-temperature stability and hot corrosion resistance along with a stress rupture life of at least 25 hours under stress of 27,500 p.s.i. at Cr; F. can be attained through (1) the control of the type of precipitation of strengthening phases first with carbon, and second with the elements titanium and aluminum in a nickel matrix, (2) the control of the solution-strengthening mechanisms as a result of the presence of W and M in particular portions to precipitate desirable carbides, along with (3) the substantial elimination of the well-known embrittling and weakening phases such as sigma and eta. Broadly, the composition which defines such an alloy consists essentially of, by weight, 0.l-0.3% C; greater than 13% but less than 15.6% Cr; 4-6% Ti; 2-4%Al; 0.0050.02%B; 3-6%W; 2.55%M0; greater than 5% up to Co; up to 0.1% Zr; with the balance nickel and incidental impurities provided that the ratio of Ti to A1 is greater than I but less than 3:1, the sum of Ti and Al is 7.5-9% Mo-l: w/2 is 5-7%.

in the alloy of the present invention, caTbon, preferably in the range of about 0.l50.2 percent provides for carbide formation which leads to improved strength particularly at high temperatures. lnsufficientcarbon, for example at about 0.08 percent is insufficient for high-temperature strength whereas an overabundance of carbon, for example, above about 0.3 percent results in lower life and embrittlcment at lower temperatures as a result of excessive carbide formation in the grain boundaries.

The element chromium provides oxidation and hot corrosion resistance. However, in amounts of less than 13 percent there is insufficient hot corrosion resistance provided in the temperature range of about 1,500-l,800 F. Cr in amountsgreater t han ir ziaarrresds'mse formation orsrgiirssna other deleterious phases without proper phase control. Accordingly, the preferred Cr range is 13.5-14.5 percent to assure such phase control.

As is the case with Cr, Co in excessive amounts can result in sigma phase formation. However, in the proper amounts described herein, Co adds to the gamma prime solubility and affects ductility of the alloy.

Very critical to the alloy of the present invention are the elements W and Mo which generally are identified with the solution-strengthening mechanism of a nickel base alloy; However, it has been recognized that a complex control of both sigma phase and of precipitating carbides can be achieved through a careful balance of the amounts of W and Mo. As will be shown in detail in connection with the specific examples, it was unexpectedly recognized that if the total amount of W and M0 were maintained such that the sum of half of the W and all of the M0 were in the range of 5-7 percent, not only could the formation of sigma phase be inhibited but also the more stable M.C carbide could be formed along with the M c, carbide rather than all M c, Although Mo has been included in substantial amounts in certain known nickel base alloys, it has been recognized that Mo on a weight )Bnd 2.5-5 percent Mo i n o rd er to maintain optimum alloy strength and stability T The elements Ti and Al have been described in connection with their function as the primary precipitation-strengthening elements in combination with nickel in forming Ni,(AlTi). Now it has been unexpectedly discovered that in the proper combination, they also function to improve hot corrosion (sulaa sssis aassr is lsrhl 2 25199? 1 .8 ns Thrs unique combination of Al and Ti along with described for Mo and W in their dual control function is one important aspect of the present invention not recognized in known nickel base alloys.

The present invention recognizes that Ti/Al ratio must be greater than 1 to provide such improved hot corrosion resistance but less than 3:] to prevent the formation of the weakening eta (Ni Ti) phase. Al is preferably included in the range of about 2.5-3.5 percent because for one reason it can result in the formation of sigma phase somewhat more readily than does Ti; Al ties up more nickel from the matrix to form the Ni ',(Al,Ti), sometimes referred to as gamma prime. This occurs because of the lower atomic weight of aluminum compared with titanium. As the gamma prime content increases, there is less nickel available in the gamma matrix. Therefore, there is a greater tendency for sigma. phase formation due to the relatively larger amounts of Cr, Co, Mo, and W in the matrix. Accordingly, it is an objective to keep as much nickel as possible in the gamma matrix.

Hence, keeping close control and lowering the Al content relative to the Ti content will result in less tendency to form the embrittling sigma phase and the higher Ti/Al ratio will im prove hot corrosion resistance.

The present invention recognizes the criticality of United States patent of aluminum and titanium not only from the standpoint of the ratio of aluminum and described above but also that at least 7.5 weight percent of the two elements is required but no more than 9 weight percent can be tolerated without seriously depleting the practiced matrix. The proper amount of Al stabilizes the gamma prime phase and prevents the Ni Ti formation. With too much Ti, the Ni (Al,Ti) is metastable and breaks down to form the weakening Ni Ti.

Although iron has been included or tolerated in certain relatively large amounts in known nickel base alloys, the present invention recognizes that iron tends to form deleterious phases. Therefore, it is preferred that no iron be present although slight adjustment such as in the solid solutionstrengthening elements can be made to tolerate small amounts of Fe.

Boron is included within the range of 0.005-0.02 percent for its beneficial effect on rupture strength and ductility. The

that just alloy including boron below that level is weak, whereas too high a boron content results in the formation of excessive borides leading to incipient melting on over temperature exposure.

It has been recognized in evaluation of the present invention that the elements Cb and Ta are not substitutes for W and Mo. It is believed that about half of the Cb or Ta goes into gamma prime formation, such as Ni (Al, Ti Cb, Ta), and to carbides. Both deplete the matrix and are undesirable in the balanced alloy defined by the present invention. Both can lead to the formation of sigma phase.

These unusual aspects of the present invention will be more clearly understood from the following detailed examples typical of alloys melted in the evaluation of the alloy of the present invention. The alloys were melted by the commercial vacuummelting techniques widely used in the preparation of nickel base alloys. Heats ranging in size from about 12 pounds to 7 about 1,000 pounds have been made, the latter being made on alloys within the range of the present invention. Test specimens were prepared either by casting directly from the melting furnace into precision casting specimen molds or by remelting and casting previously prepared alloy ingots.

The alloy forms representative of those melted within the scope of the present invention are shown in the following table TABLE 1 [Composition in weight percent] Alloy includes 0.0140.016% B; 0.03%I Zr, balancev Ni and incidental C Or M0 W Ti Al Other alloys made and tested during the evaluation of the alloy of the present invention include those shown in the following tabie 11, outside the scope of the present invention.

TABLE II [Composition in weight percent] grain boundary M C carbides. Then when all the available carbon is thus reacted, it appears that excessive chromium in the matrix combines with such elements as Co, Mo, etc. to form a Cr-Co-Mo type sigma. Long time stability testing such as at 1,500 F. at a stress of 55,000 p.s.i. identifies the strength-reducing nature of sigma phase. The terminal nature of such sigma phase has been reported by Boesch and Slaney in Metals Progress, July 1964, pages 109-11 1.

'Although sigma phase may be removed by heat treatment, it will recur when the alloy experiences the same time and temperature conditions under which sigma was originally formed. The alloy of the present invention identifies a different kind of alloy which inhibits original sigma phase formation and results in the improved combination of higher temperature strength and stability along with hot corrosion resistance as a result of a different surface reaction product.

In order to understand more fully the present invention and its individual components as they affect the strength and stability of the alloy of the present invention, the following tables have been prepared. These compare the alloy forms both within and outside the scope of the present invention, as shown in full compositions in tables 1 and II. The element content referred to in the tables as well as throughout this specification is in weight percent and the term "ksi refers to thousands of pounds per square inch.

TABLE III As-cast stress rupture life (hrs) Element variation, 1,500 F./ 1,800 F./ Wt. percent 65 K 5.1. 27.5 K 5.1. Sigma 000 230 None. 5G0 481 Do. 10.0 C0 744 49 Do. 12.3 C0 896 50 Do. 15.0 00 836 33 Small 1513 Gr 58G 38 Large 13.9 Cr 744 49 None 13.0 Cr 588 30 Do 0.08 C 512 Small 0.18 C 896 None. 8 0.26 C 670 43 Do.

Heat treated: 2,200 F. 2 hrs.; 2,000 F.-4 hrs.; 1,550 F.-16 hrs.; 1,400 F.10 hrs.

Alloy includes 0.0140.017% B; 0.03 0.04% Zr with the balance Ni and incidental impurities 2 C Cr 00 Mo W Ti Al 'Ii/Al Ti+A1 Mo+Wl The improved characteristics of the present invention are particularly measured by combination of high-temperature stress rupture life and stability along with hot corrosion resistance. This improvement as it relates to longtime stability is related to the suppression of the formation of such embrittling phases as sigma and eta. These phases are greatly suppressed or are entirely eliminated according to the alloy of the present invention. When certain known case alloys are exposed to elevated temperatures, the gamma phase and carbides, which are found in the primary gamma prime phase, agglomerate. At temperatures in the range of about 1,300 F.-l ,800 F., sigma plates formin matrix areas surrounding the gamma prime. This formation, which is accelerated by stress, appears to relate to excessive chromium in the primary gamma prime and surrounding matrix areas, first reacting with carbon to form As shown in table 111, in the alloy of the present invention, cobalt below about 15 weight percent does not lead to the forrnation of sigma phase. Although at 15 percent Co, a small amount of sigma is beginning to form, this can be tolerated as the upper limit because the strength and stability characteristics have been reduced only a slight amount. However, above about 15 weight percent, excessive sigma will form resulting in a different kind of alloy of reduced properties. Because the alloy formed at 5 percent Co was significantly weaker than desired, the higher temperature tests were not run.

With respect to the chromium variation shown in table 111, the detrimental effect of the formation of heavy amounts of sigma on longtime stability is shown by alloy 14 at 15.6 percent Cr. The identification of large amounts of sigma shows alloy 14 to be of a different kind than that of alloy 6 within the scope of the present invention. Alloy at 13 percent Cr and only 0.9 percent lower than alloy 6, shows a reduction in strength even though all the elements of alloy 15 are within the range of the present invention. Therefore, the alloy of the present invention includes less than 15.6 percent but greater than 13 percent Cr.

With respect to the carbon variation in table Ill, it can be noted that at 0.08 percent C, insufficient carbon is present to react with Cr within the range of the present invention to prevent Cr from forming sigma platelets. The reduction in long time stability as represented by the 1,500 F. tests should be noted in this regard. Although amounts of carbon approaching about 0.3 percent can be included, it is preferred that carbon at about 0.2 weight percent be maintained in order to assure the unusually fine properties of the preferred form of the alloy of the present invention.

Although the elements W and Mo have been included in known nickel-base alloys singly or interchangeably as solution-strengthening elements, the present invention recognizes additional critical roles played by these two elements. Both are involved in the complex control of precipitating carbides and sigma phase formations, although M0 is a more potent sigma phase former. The following table IV shows the effect M and interrelationship of these elements on the alloy of the present invention.

TABLE IV except for the element variation listed, which in the case of alloy 19 is Mo and in the case ofalloy 20 is (MO+W/2).

The elements Ti and Al contribute to the alloy of the present invention in several ways. This invention recognizes that the proper amount and interrelationship between these elements can control the short time strength, the alloy stability through sigma phase inhibition and, very importantly, provide hot corrosion resistance.

The problem of hot corrosion resistance involves resistance to sulfidation in the range ofabout l,500- 1 ,800 Above and below that range, hot corrosion resistance is not as significant a problem in the type of alloys to which the present invention relates because such alloys include the element aluminum. Aluminum oxide which forms on the surface as a reaction product will form a reasonably protective oxidation resistant barrier. The problem of oxidation resistance is different from that of hot corrosion resistance. Normally for oxidation resistance it would be better to have a Ti/Al ratio of greater than 1. The higher ratio is desirable because TiO, is formed on the surface. The more TiO available. the better is the hot corrosion resistance. However, Ti in amounts which would produce a Ti/Al ratio of about 3:1 or more, cannot be tolerated in the alloy of the present invention.

The efifect of the elements A] and Ti on the alloy of the present invention as it relates to as-east stress rupture life and stability is shown in the following table V.

Element variation As-cast (weight percent) stress rupture life (hrs.)

1,500 F./5 5 1,800 F./27.6 Alloy M0 W (Mo+W/2) 1 s.i. k. s.i. Sigma 1 10 6.1 3.0 7.6 (0 21 Medium. 6 3.0 6.0 6.0 744 49 None. 20 4.9 4. 9 7.4 43'.) 47 Medium. 9 2. 9 5. 0 5.4 746 37 None.

Based on a 200 hour life 1,500 F68 k. s.i. rupture test.

TABLE V Ascast stress rupture life (his) Weight percent Ti Al ('ITIITABM Ti/Al 65ks.i. 21.5 ltsj. Sigma as 4.0 7.8 1.0 309 as Medium 2.9 5.0 7.9 0.6 251 25 Large. 4.6 2.6 7.1 1.7 560 19 None. 5.0 3.0 8.0 1.7 896 00 Do.

In alloy l9,even with Mo as high as 6.1 percent, there is in sufficient strengthening to provide adequate high-temperature stress rupture strength. More importantly, however, is the fact that the total amount of Mo and W is sufiiciently high to result in sigma phase formation as measured by the atomic relation ship between those elements of(Mo+W/2)of as high as the 7.6 percent The present invention contemplates that relationship to be within the range of 57 percent to inhibit sigma phase formation and precipitation of the proper carbides as described before. Alloy 20, a different kind of alloy and outside the scope of the present invention, includes Mo and W within the invention range but with the improper relationship one to the other as shown by the(Mo+W/2)of 7.4 percent.

The formation of medium amounts of sigma resulted in significantly reduced stability as measured by the l,500 F. stress rupture test. Alloy forms 6 and 9, within the scope of the present invention, have the proper balance of W and Mo and are a different kind of alloy because of the absence of the sigma structure. This results in improved stability and strength.

In the above tables lll and IV, it should be noted that the alloy forms identified with numbers greater than 10 have compositions within the range of the alloy of the present invention Although alloys 5, 1 l and 12 include about the same amount of the sum of titanium and aluminum, it should be noted that alloy 5 forms no sigma phase whereas alloys 1 l and 12 form medium to large amounts of sigma phase. This can be attributed to the improper relationship between the two elements. The fact that different kinds of alloys are formed between alloy 5 and alloys 11 and 12 is further substantiated by the stress rupture life, particularly the stability data represented by the l,500 F. tests. Further, it should be noted that the alloy 13, although having the proper ratio between Ti and A l does not have sufficient amounts of these elements to provide the required strength. Therefore, the alloy of the present invention defines the relationship between Ti and Al such that the sum of those elements is in the range of about 7.59 percent and that the Ti/Al ratio is greater than one but less than 3:1.

One important characteristic of the alloy of the present invention which distinguishes it from known alloys presently intended for the same use is its significantly improved hot corrosion resistance. A series of comparison tests to determine the hot corrosion resistance of a variety of alloys was conducted on such known nickel base super alloys as those listed in the following table VI.

TABLE VI [Known alloys in weight percent] Alloys include .0l.02 B, balance Ni and incidental impurities C Cr Mo W Ti Al Zr Others 6.0 7. 2.0 6.0 1.0 5. 5 l. 3 9 TB, 0.5 Hf, 0.5 Cb,

0.5 Re. 15.0 22.0 4.4 2.4 4.4 13. 0 4. 5 0. 75 6.0 10 2.3 Cb-i-Ta Because the alloys tested were intended for use in a gas tur- 400 hours at elevated temperatures. The results of one such bine engine, test apparatus simulating conditions in the turparis is h n in the l ing table I bine section of a gas turbine was constructed. The apparatus burned jet fuel, for example JP-S in a 30-1 air-fuel mixture and injected sea water having a composition within the range of ASTM specification D-665-60. The sea water was diluted with distilled water to fiv parts Per million. The tests run were ,700" F. 1,s00 F. cyclic tests over a period of 1,000 hours including 18 intermit- Alloy; tent colling cycles to room temperature with an air blast. The 20 2 Tioz (s) plus (M) Tioz (Mlplus (M) EXPOSURE TABLE IX.X-RAY DIFFRACTION DATA AFTER 400 HRS.

specimens tested were cast bars ground to a diameter of about B m ii ii'i i si iiiiiihho. (W) iii iiiit gs i g hig ithoi 0.130 inch and about 1.25 inches long. Results of such a com- Plus Tioz Tioz parative test are shown in the following table VIl. s) tmn (M) =niedium, (W) =weak, (V) =vcry.

TABLE VI! Hm Resimnce 1000 cyclic Alloy 2 within the scope of the present invention and having Dcmh f Attack (Mus). a remarkable resistance to hot corrosion had a substantial amount of TiO in its surface reaction product. Only a small [60F 3 amount of that oxide is found in the reaction product of the MIG/I2 alloy B. Thus the two alloys are ofa different kind. /44/41 26/36/44 What s ed ("1 (a 1. A cast nickel base alloy of improved stability, strength 53 213%? and corrosion resistance, consisting essentially of, by weight:

n 30/36/39 3 5 about 0. 15-03 percent C, said carbon percentage being greater than that required for deoxidation and in addition Expressed as l/2 (surface loss/avg. penetration/max. penetration). being sufficient for forming grain boundary carbides; d spec'men corroded 'hroughwt' greater than 13 percent but less than 15.6 percent Cr; From table Vll, it is easily seem that at all temperatures greater than 5 p to 15 P C0; tested, alloy 2 within the scope of the present invention is re 40 Percent markably more resistant to hot corrosion than are all of the P f other tested known alloys, most of which are presently in use Percent in the hot section of gas turbine engines. Percent Another measure of hot corrosion resistance involved a 0-O050s02 P B;

study of specimen weight loss rather than surface penetration P to about Percent? or thickness loss. Another series of tests resulting in data of the balance essem'auy mckel and mmdemal lmpum'es which the date of table VIII is a typical were performed on althe Ti/A being 8mm less than 3:1-

I t loys both within and outside the scope of he presen inven the Sum of Ti and A1 being in the range of 75-9 p tion.

and

TABLE VI" the sum of Mo and half of the W being in the range of 5-7 Hot Corrosion Resistance 1750Ffor 500 hrs. percent; and further characterized by the substantial 1 absence of sigma phase and a stress rupture life in the as- Loss S, cast condition of at least about 25 hours under a stress of 0,055 Max, 27,500 psi. at ,800 F.

2. The alloy of claim 1 in which:

2- 2- the C is 0. l 5-0.2' percent; the Cr is 13.5-14.5 percent; 3. 10. the Co is 7.5-l2.5 percent; the M0 is 3.5-4.5 percent;

the W is 3.5-4.5 percent; r the Ti is 4.5-5.5 percent;

Alloy 10 within the scope of the present invention shows significant and remarkable resistance to weight loss after 500 hours at l,700 F. as compared with alloys known or outside the Al is 2.5-3.5 percent; the B is 0.01-0.02 percent;

the scope of the present invention. the ls l P f and the Ti/Al ratio is 1.1-2.1

The significantly improved hot corrosion resistance of the 3 The alloy f claim 2 in which; alloy of the present invention is based on the fact that it is a the Cr is 3 7 4 3 percent; different kind of alloy. Hence a different kind of reaction h c i 9.10 percent; product is formed on the surface of the alloy of the present inh M i 3 7 4 3 percent; vention under oxidizing conditions than is formed on the surh w i 1743 ent; faces of certain known nickel base alloys intended for the the Ti is 4.8-5.2 percent; same purpose. As an example of such difference, an X-ray difthe Al is 2.8-3.2 percent;' and fraction study was made on such surfaces after exposure for the Zr is 0.02-0.04 percent. 

2. The alloy of claim 1 in which: the C is 0.15-0.2 percent; the Cr is 13.5-14.5 percent; the Co is 7.5-12.5 percent; the Mo is 3.5-4.5 percent; the W is 3.5-4.5 percent; the Ti is 4.5-5.5 percent; the A1 is 2.5-3.5 percent; the B is 0.01-0.02 percent; the Zr is 0.005-0.1 percent; and the Ti/A1 ratio is 1:1-2:1 .
 3. The alloy of claim 2 in which: the Cr is 13.7-14.3 percent; the Co is 9-10 percent; the Mo is 3.7-4.3 percent; the W is 3.7-4.3 percent; the Ti is 4.8-5.2 percent; the A1 is 2.8-3.2 percent; and the Zr is 0.02-0.04 percent. 